A cache is a buffer between a bottom storage device (e.g., hard disk) and an external interface. The cache may store for example hot data to reduce data exchange between the external interface and a bottom drive, improve data transmission rate and reduce delay of data access. The cache has a fast access speed, and furthermore, a magnitude and speed of the cache are important factors that will affect transmission rate of a bottom drive.
A fast cache works as a layer 2 (L2) cache, which contains copies of logical blocks resident on bottom physical drives. As an example, the fast cache may be built up by independent solid-state disk (SSD) pairs, which makes it easy to expand and shrink the fast cache. At present, there is still room for improving a storage system including a plurality of cache devices in terms of its architecture, management as well as overall performance.